In some prior art disk drive servos, a calibration system referred to as sine calibration, or sinecal, is used to calibrate the system gain. The sinecal method operates with a closed loop servo system having a system control signal, a system gain and an actuator. In operation, the sinecal calibration system injects a single frequency sinusoid signal into the servo system so that the sinusoid signal is added to the system control signal. Then the control signal and the added sinusold signal are multiplied by the sinusoid signal to generate a first product signal. The actuator position is measured. Then the measured actuator position is multiplied by the sinusoid value to generate a second product signal. The first product signal is low pass filtered to generate a first filtered signal, and the second product signal is low pass filtered to generate a second filtered signal. The second filtered signal is divided by the first filtered signal to generate an identification ratio which is proportional to the measured system gain.
A new calibrated system gain adjustment factor is determined as a function of the identification ratio. The previous system gain adjustment factor is replaced by the new calibrated system gain adjustment factor. The control signal is multiplied by the new calibrated system gain adjustment factor to obtain an adjusted control signal. The result is that the control system automatically compensates for bias forces and adapts to changes in effective system gain.
The sinecal system described above is more than satisfactory for the DASD for which it was designed. Ordinarily, the DASD is powered up at the beginning of the work day, calibrated, and operated all day until powered down. Smaller computer platforms that are battery operated operate in a different mode. As a power saving measure, the smaller computer platforms are powered down many times during the work day, and are re-calibrated every time they are powered up. The smaller computer platforms use smaller drives, and it has been found that the sinecal calibration system is not appropriate for these smaller drives. During the early development of a smaller disk drive, attempts were made to use the sinecal method of calibration. Sinecal had several problems in the smaller head disk assembly (HDA).
The principal problem with sinecal is the time required for measurement. It was taking 1.5 seconds for each measurement and the new smaller HDA required two measurements. This meant a calibration time of three seconds, which was unacceptable for this product if repeated re-calibration occurred, as would be the case with power saver features causing power down throughout the work day.
Other HDA's also would require more calibrations due to increases in the number of heads and the addition of gain zones to account for gain changes as a function of radial head position on the disk. Gain zones increase the number of calibrations for individual heads because each head requires one calibration for each zone. The maximum number of zones was set at six, and the maximum number of heads at eight. An HDA with eight heads and six zones would require 48 calibrations. Since each calibration requires 1.5 seconds, these 48 calibrations would take 72 seconds, assuming no retries. Significant reduction in the calibration time was needed.